1. Field of the Invention
The present invention relates to a novel color reproducing method adaptive to a recent open image data processing system, that is, an image data processing system consisting of systems and equipments each of which is independently specialized to its function.
More specifically, this invention relates to a novel color reproducing method developed to overcome the biggest problem retained in such an open image data processing system in which faithful and accurate color reproduction is difficult since each of the systems and equipments making up the open image data processing system is distinctively organized.
2. Description of Prior Art
The trend in the image data processing system is that systems or equipments forming the image data processing system such as a color input device (such as imagesetter, output scanner, etc.), a platform (such as image data processing unit or CPU unit of a computer) and a color output device (such as imagesetter, output scanner, etc.) are independently specialized to respective functions. As a consequence, such an open image data processing system can be made up of systems and equipments manufactured by not only the same maker but also different makers.
A desk top publishing system (DPT) is a typical example of such an open image data processing system. In actual circumstances, a user of the desk top publishing system combines an input device, an output device and the like at will to make up the system, in order to adapt the system to the actual conditions of the user's work. A feature of the desk top publishing system is a high versatility allowed when the user selects component systems and equipments and combines them to form the whole system.
As is well known, each data such as character, graphics, image, or the like are recognized and processed on the same dimension as computer data. PostScript (tradename) is a computer language that is mainly used for processing of such data. It is said that the spread of PostScript as a standard language contributed to a wide practical use of the desk top publishing system to a great extent. In the desk top publishing system, image data are described in said PostScript language and stored in PostScript format. When the image data are outputted, the computer interprets the image data in PostScript language, outputs and develops them. In 1992, the desk top publishing system started to bypass the conventional plate-making work and to complete the process from input to output. For example, a part of a color printed magazine was made through a plate-making system relying on a desk top system. This was said to be the arrival of practical use of the desk top system (p.11-26, March, 1993, Japan Camera Society Journal, Japan Camera Society).
In an open system made up of various sorts of systems and equipments, a particular problem is how to keep quality of an image, more specifically, how to maintain the color reproducibility among the systems and equipments at a constant level or to maintain consistency of the color. This is natural since in a product resulted from a desk top publishing system, the quality of image portions is appraised on a level quite higher than character portions.
In consequence, there is an attempt to introduce a color management system to an open image data processing system as seen in the desk top publishing system to form a so-called open color system that is opened with respect to even the processing of color. To cite an example, Adobe Systems adopts a manner to provide a converting function for a color space to an output system to download a desired conversion algorithm or a lookup table (LUT) from a host computer to the output system. Incidentally, it is a matter of course that said conversion function can comply with a plurality of color spaces in accordance with an output form of the output system, for example, a printed image of a laser printer in a CMYK system or a CRT image including a liquid crystal display image in RGB system.
As a color space, which is a base of said color management, there have been proposed various color systems by CIE (Commission Internationale de l'Eclairage) such as the RGB color system, XYZ color system, and Lab or Luv system obtained by mathematically converting (i.e., by matrix conversion of) the XYZ color system. Such color systems do not accurately reflect actual colors (that is, color perception) from limitation of colorimetry conditions (such as field angle, employed light source etc.). No uniform standard has been found yet, and the research has been thus continued.
From a view to establish a color reproducing method independent of the various systems and equipments, which make up an open image processing system, a concept of "device independent color" (DIC) that means a color reproducing method being effective independently of systems and equipments has come to be important. As one way to establish said DIC, there is a proposal to introduce color system conversion where data is converted, for example, from RGB to XYZ, then from XYZ to Lab by the matrix conversion as described above, and data converted into Lab is used as common image data in the systems and equipments. This proposal is, however, not sufficient since the various color spaces proposed by CIE have restrictions and limitations as mentioned above.
If the above proposal is adopted as the second best policy, the following shortfalls and disadvantages can be easily noticed in the practice of actual image data processing work. Such shortfalls and disadvantages are:
(1) If picture image information (density information) is obtained by means of a given input scanner from a color original or form, as a typical example, a color film original (of a transparency type) admitted that an object has been faithfully photographed (or recorded) thereon, said picture image information is, nevertheless, dependent of characteristic curves (also called as, photographic characteristic curves of R, G and B) of a film that has been used upon photographing the object. PA1 (2) The above situation is the same if using the same film, unifying the photographing conditions to make a standard original, and processing the image data of each non-standard image on the basis of image data of the standard original as a reference. PA1 (3) There are used various films to take an original thereon, each of which has its own characteristic curves (i.e., photographic characteristic curves). Under such circumstances, image data in Lab admitted by CIE as image data obtained from a standard original taken on a given film is useless as common data (or a reference base). PA1 (4) In a given input scanner system, conversion from RGB color system to XYZ color system is, in general, conducted by the following formula 1 or formula 2; ##EQU1## where SCR: a calibration function of a scanner; and PA1 .gamma.: a correction value. PA1 In the case of a color output device (such as imagesetter and output scanner) used in an open image data processing system such as a desk top system, such color output devices used to output, for example, halftone images have different and various tonal conversion formulae. These tonal conversion formulae used to form output images are different depending on the device, thus lacking uniformity among them. As is well known, a dynamic range of a color film original (in RGB system) largely differentiates from that of an output image (in CMY system). As a consequence, it is necessary to rationally compress the tone (including color tone and density gradation) or convert the tone (i.e., conversion from a continuous tone to a halftone) when producing a halftone picture. At that time, unified adjustment of color tone and density gradation is dispensable upon compression and conversion of the tone. The conversion of density gradation in existing output systems, which is an especially important process, has been carried out on the basis of experiences or intuitions. This is hardly rational. PA1 (1) to a color input device to which color picture image information is inputted, providing functions to obtain and retain light intensity values, which are picture information, using characteristic curves, i.e., photographic characteristic Curves Of a film that has been used for a color film original when picking up the picture image information from the color film original, said color film original is a typical example as an original in the art; and PA1 (2) to a color output device serving to output the picture image information, providing functions to convert the tone (including color tone and density gradation) of the color film original using a specific tonal conversion formula, then output them. With the above technical constitution, the shortfalls of the conventional system can be overcome and color reproducibility of color (consistency of color) can be maintained in the system or among the systems. PA1 x: a basic light intensity value shown by x=(xn-x.sub.H), that is a difference between a picture information value (xn) correlating to a light intensity obtained from a density value (Dn) of a given pixel at n point in the color film original through a density characteristic curve defined in a D-X orthogonal coordinate system and a picture information value (x.sub.H) correlating with light intensity corresponding to a density value (D.sub.H) of the brightest part (H part) of the color film original 5, a D axis of the D-X orthogonal coordinate system representing density of the color film used upon photographing the color film original 5 and an X axis of the D-X orthogonal coordinate system representing picture information values correlating with light intensity; PA1 y: a tonal intensity value of a pixel in a reproduced picture corresponding to the given pixel at the n point of the color film original; PA1 y.sub.H : a tonal intensity value set in prior to the brightest part of tile reproduced picture corresponding to the brightest part of the color film original 5; PA1 y.sub.S : a tonal intensity value set in prior to the darkest part of the reproduced picture corresponding to the darkest part of the color film original 5; PA1 .alpha.: a surface reflectance of a recording medium of the reproduced picture; PA1 .beta.: a value determined by .beta.=10.sup.-.tau. ; PA1 k: a value determined by k=.gamma./(x.sub.S -x.sub.H), where x.sub.S is a picture information value correlating to light intensity determined from the density value (D.sub.S) of the darkest part of the color film original 5 through the density characteristic curve defined in the D-X orthogonal coordinate system; and PA1 .gamma.: a given coefficient.
It is, however, doubtful that a unified function or .gamma. value can be commonly employed in various input scanner systems. If possible, said shortfall, that is, a problem of dependency on characteristic curves of a film still remains.
An object of this invention is to provide a novel method to establish DIC (Device Independent Color, more concretely, a color reproducing method independent of systems and equipments), which is said to be difficult in the image processing work in an open image data processing system such as a desk top systems.
The object of this invention is to establish DIC in an open image data processing system and is accomplished by: